Inductively keyed control circuits utilizing rf energy coupled to a keying station accessible to an external keying circuit, have been disclosed in prior patents. For example, U.S. Pat. Nos. 3,723,967 and 3,842,324 disclose single-frequency systems depending on rf absorption in an external keying circuit to enable the generation of a control output signal. Co-pending patent applications Ser. Nos. 657,760, 652,465, 660,116 and 675,950 disclose the use of multiple keying frequencies. All of the previously disclosed systems make the sensing coil a significant part of the oscillator tank circuit. When implemented in this way, the provision for unlocking stations at a plurality of locations required to plurality of oscillators and/or associated detectors. In addition, prior swept-frequency devices required the use of a dead oscillator detector in order to avoid keying by a broadband absorber such as iron.